(1) Field of the Invention
This invention relates to a III nitride epitaxial wafer and the usage of the same, particularly to a III nitride epitaxial wafer and the usage of the same preferably usable for a substrate of semiconductor devices such as a light-emitting diode or a high speed IC chip.
(2) Related Art Statement
III nitride films are used as semiconductor films for light-emitting diodes, and recently get attention as semiconductor films for high speed IC chips. Particularly, Al-including III nitride films get attention as field emitter materials.
Such III nitride films are formed on an epitaxial wafer, for example, which has an underfilm epitaxially grown on a given base material. The underfilm is preferably made of Al-including III nitride films in order to facilitate the epitaxial growth of the III nitride films to be formed. Moreover, such Al-including III nitride films formed between the base material and the III nitride films can enhance the efficiency of a semiconductor element because of a wider bandgap of Al-including III nitride material.
The epitaxial wafer is set on a susceptor installed in a given reactor, and heated to a predetermined temperature with a heater built in or out of the susceptor. Then, a raw material for a III metal, nitrogen and, if necessary, other elements are introduced into the reactor with carrier gases, and supplied onto the epitaxial wafer to fabricate the III nitride films by an MOCVD method.
However, the fabrication process of the epitaxial wafer is carried out in a different apparatus from the one where the above-mentioned fabrication process of the III nitride films is done, partly because the fabrication condition of the epitaxial wafer is different from the fabrication condition of the III nitride films. Therefore, the epitaxial wafer is exposed to the environmental atmosphere after the fabrication process of the epitaxial wafer. Moreover, there is a problem with damages forming at the surface of the epitaxial wafer when characterizing the epitaxial wafer.
Then, the surface of an Al-including III nitride underfilm of the epitaxial wafer is oxidized and/or comes to include some defects. As a result, the III nitride films can not have good crystal quality due to the bad condition of the epitaxial wafer surface.
From this point of view, such an attempt was made that an Al-including III nitride films as an underfilm is formed thicker on a base material, and the oxidized surface layer of the underfilm is removed. However, the oxidized surface layer including Al can not be removed by hydrogen gas or ammonia gas, which are used in the fabrication of a III nitride films, and accordingly, another etching apparatus are required. As a result, the whole fabrication process becomes complicated, and the operationality and the yield ratio are deteriorated.
It is an object of the present invention to prevent the oxidization of an under film made of an Al-including III nitride film of an epitaxial wafer, and to the usage of the epitaxial wafer having an underfilm that is not oxidized.
In order to achieve the above object, this invention relates to an epitaxial wafer comprising a base material made of a single crystal material, a III nitride underfilm including at least Al epitaxially grown on the base material, and an InaGabN (a+b=1) film formed on the underfilm.
This invention also relates an epitaxial wafer including a base material made of a single crystal material, a III nitride underfilm including at least more than 50 atomic percentages-Al element epitaxially grown on the base material, and an InaGabN (a+b=1) film formed on the underfilm.
The oxidized surface layer of the InaGabN film is removed through an etching process, and a III nitride film is formed on the InaGabN film, in which the thickness of the epitaxial wafer has been reduced subsequent to the etching process.
The inventors intensely studied ways to prevent the oxidization of the Al-including III nitride underfilm. As a result, they discovered that a protective film should be fabricated on the underfilm. However, it was required that the protective film did not obstruct the epitaxial growth of the III nitride films, and the oxidized surface layer of the protective layer needed to be easily removed. Accordingly, it is very difficult to match suitable materials to the protective film.
Then, the inventors made enormous research and development to discover suitable materials for the protective film. As a result, the inventors discovered that if the protective film is made of an InaGabN film (a+b=1), the epitaxial growth of the III nitride film is not obstructed and the oxidized surface layer of the protective film can be easily removed by an etching process using hydrogen gas and/or ammonia gas, which are used in the fabrication process of the III nitride films.
The InaGabN film functions as an underfilm for the III nitride films as well as the protective film to be formed. Therefore, the InaGabN film can be fabricated on the similar condition to the one of the Al-including III nitride underfilm. Therefore, both of the InaGabN film and the Al-including III nitride underfilm can be made by the same apparatus, so that the whole fabrication process can be simplified.
On the above-mentioned enormous research and development, the inventors found out that the InaGabN film is extremely suitable for the Al-including III nitride underfilm, and attained the epitaxial wafer and the usage of the substrate according to the present invention.
Herein, the Al-including III nitride underfilm is constructed, as occasion demands, of a single layer, a multi-layered structure or a composition-gradient film in which the composition of a given element is inclined in its thickness direction. Therefore, the Al content of the Al-including III nitride underfilm means the one over the underfilm structure.
The InaGabN film may contain inevitably, in addition to In, Ga and nitrogen, H, O, C, Al, Si, Ge, Mg, Zn, Be or the like, depending on the film-forming condition, the raw materials or the reactor materials.
Moreover, the InaGabN film may be entirely removed for simplifying the etching process, instead of removing the oxidized surface layer of the InaGabN film. The etching process for the InaGabN film does not influence the properties of the III nitride film to be formed on the epitaxial wafer.
This invention will be described in detail, hereinafter.
The epitaxial wafer is required to have the InaGabN film on the Al-including III nitride underfilm. The InaGabN film can be made under similar conditions and with the same apparatus used for forming the Al-including III nitride underfilm, as mentioned above. For example, trimethylgallium as a raw material for Ga and ammonia gas as a raw material for nitrogen are supplied on the base material of the epitaxial wafer heated to 1050xc2x0 C. to fabricate the InaGabN film.
Although the thickness of the InaGabN film is not restricted, it may be preferably set to 50 xc3x85 or more, particularly 100 xc3x85 or more. As the result of the detailed investigation for the oxidization mechanism of the InaGabN film, the oxidization proceeds to a depth of about 50 xc3x85 from the surface of the InaGabN film at the maximum case. Therefore, if the thickness of the InaGabN film is set within the above-mentioned thickness range, the Al-including III nitride underfilm positioned under the InaGabN film is not substantially oxidized.
Although the upper limit thickness of the InaGabN film is not particularly restricted, it may be preferably set to about 1 xcexcm or below. If the InaGabN film is fabricated thicker than 1 xcexcm, the properties of the epitaxial wafer can not be improved, and instead, some cracks and exfoliation are created in the film due to the stress in the grown film.
The InaGabN film is preferably a GaN film. In this case, the protective effect for the Al-including III nitride underfilm can be enhanced.
It is required that the Al-including underfilm includes at least Al element, preferably Al content of 50 atomic percentages or more, for the enhancement of the efficiency of a semiconductor element, including the epitaxial wafer and the III nitride film. The surface oxidization of the underfilm is caused mainly due to the Al element of the underfilm. Therefore, although the underfilm having a large Al content suffers from the surface oxidization conspicuously by nature, the real surface oxidization can be repressed by the protective film made of the InaGabN film.
Although the thickness of the Al-including III nitride underfilm depends on the kind and the crystal quality of the III nitride film to be formed, it is preferably set to 0.5 xcexcm or over, particularly within 1-3 xcexcm. For the enhancement of the crystal quality of the III nitride film to be formed, it is desired that the underfilm is formed thicker, but if the thickness of the underfilm is beyond 3 xcexcm, some cracks and exfoliation are created in the underfilm.
In the case of fabricating a light-emitting diode or the like, some semiconductor films having their respective compositions of AldGaeInfN (d+e+f=1) are formed on the epitaxial wafer. In this case, therefore, it is desired that the Al-including III nitride underfilm has the similar composition of AlxGayInzN (X+y+Z=1, Xxe2x89xa70.5), particularly AlxGayInzN (X+y+Z=1, Xxe2x89xa70.7). More particularly, the underfilm is constructed of an AlN film. Thereby, the epitaxial growth of the III nitride film to be formed on the underfilm is much developed, and thus, the crystal quality of the film can be more enhanced.
As mentioned above, if the underfilm includes a large Al content, the surface oxidization of the underfilm can be repressed by the protective film made of the InaGabN film according to the present invention.
The Al-including III nitride underfilm may contain B, Si, Ge, Zn, Be, Mg or the like. Moreover, the underfilm may contain inevitable minute amounts of other elements and minute impurities depending on the film-forming condition, the raw materials and the reactor material, in addition to the above-mentioned intentional elements.
The base material of the epitaxial wafer is not restricted. As the base material, there are exemplified oxide single crystals such as sapphire single crystal, ZnO single crystal, LiAlO2 single crystal, LiGaO2 single crystal, MgAl2O4 single crystal, or MgO single crystal, IV single crystal or IV-IV single crystal such as Si single crystal or SiC single crystal, III-V single crystal such as GaAs single crystal, AlN single crystal, GaN single crystal or AlGaN single crystal, and boride single crystal such as Zr2B2.
Particularly, sapphire-SiC single crystal is preferably employed because the base material made of the single crystal can have good physical compatibility for the III nitride underfilm having a larger Al content, and thus, provide good contact and epitaxial growth property to the underfilm.
In using the sapphire single crystal, it is desired that the main surface of the sapphire single crystal, onto which the Al-including III nitride underfilm is formed, is nitrided. In this case, a surface nitride layer is formed at the main surface, and thus, the crystal quality of the Al-including III nitride underfilm can be enhanced through the surface nitride layer. As a result, the crystal quality of the III nitride films to be formed having, e.g., a composition of AldGaeInfN (d+e+f=1) can be enhanced.
Moreover, if the Al-including III nitride underfilm is formed thicker on the main surface via the surface nitride layer, some cracks and exfoliation is not substantially created in the underfilm. Therefore, depending on the film-forming condition, the underfilm can be formed as thick as 3 xcexcm or so. Therefore, on the multiplier effect of the enhancement of the crystal quality of the underfilm due to the surface nitride layer and the thickness increase, the crystal quality of the underfilm itself can be much enhanced, and thus, the crystal quality of the III nitride film to be formed on the underfilm can be more enhanced.
The above nitriding treatment can be performed by setting the sapphire single crystal in a nitrogen-containing atmosphere such as ammonia gas atmosphere and then, heating the single crystal for a given period. The thickness of the surface nitride layer depends on the nitrogen concentration, the nitriding temperature and the nitriding period. It is desired that the thickness of the surface nitride layer is set to 1 nm or below, or as thick as the nitrogen concentration of the layer is 2 atomic percentages or over at a depth of 1 nm from the main surface of the sapphire single crystal to be nitrided.
In use of the thus obtained epitaxial wafer, the oxidized surface layer of the top InaGabN film is etched and removed by a general-purpose dry-etching or wet etching.
Since the whole fabrication process of the III nitride films can be simplified, the oxidized surface layer is preferably removed by the dry-etching.
In the dry etching, a given etching gas is introduced into the reactor, and supplied onto the epitaxial wafer heated to 900xc2x0 C. or over. Then, the oxidized surface layer is removed by the heated and/or resolved etching gas.
It is desired that hydrogen gas as a carrier gas or ammonia gas as a nitrogen supply source, which are used in the fabrication process of the III nitride films, is employed as the etching gas because the oxidized surface layer can be easily removed by such an etching gas. In this case, another chorine-based corrosion gas or the like is not required, so that the III nitride film is never corroded by such a corrosion gas in the fabrication process.